Some fuel injection systems have been designed as unit injectors which incorporate an hydraulically driven pressure intensifier with a stepped plunger for injecting fuel into the engine's cylinder and the fuel delivery and timing are controlled by an electronically controlled valve, also the spray pattern is controlled by means of modulating the base fuel pressure supplied to the unit injector. The present invention is similar to these unit injectors but improvements are added which are described herein which increase the injection pressure, decrease the amount of hydraulic energy required to drive and control the fuel injection system, improve the stability of fuel delivery in consecutive injections, reduce the minimum fuel delivery, allow for control of an injection pressure curve of the unit injector and improve its reliability. The present invention preferably also provides a method of reducing the noise level emanating from the engine.
The present invention concerns hydraulically actuated electronically controlled unit injection (HEUI) systems which are well known to the addressee. The closest art known to the present invention is that of SU-A-1671938, the contents of which are incorporated herein by reference.
In a HEUI system, there is no cam for injection purposes and the fuel is supplied to the injectors under high pressure. The high pressure varies by means of a control signal from an engine management system and a top pressure may be 200 bars or around 3,000 psi and a bottom pressure could be 500 psi. The pressure is intensified within the injector. The fuel is then metered electronically and injected into the cylinder at pressures up to 27,000 psi or around 1800 bar.
The differences between the injector and injector system of the present invention and that of the Soviet specification mentioned above comprise firstly the inclusion of resilient means to bias an hydraulically controlled differential valve to its closed position; secondly, the inclusion of a throttling slot displaying the required characteristics. The Soviet specification reveals an hydraulic differential valve where the poppet end of that valve can close off the flow of fuel but in the present invention that part of the poppet and surrounds form a throttling slot with characteristics which alter the flow of fuel and alter the parameters under which the poppet will open or close. Specifically, the throttling slot provides a restriction such that the pressure in the poppet chamber is higher than the pressure in the working chamber in the injection part of the cycle and during the metering part of the cycle the throttling slot is designed to bring about a pressure difference which holds the HDV closed. The HDV in the Soviet design cannot carry out those functions due to the lack of a throttling slot and the lack of a by-pass channel between the control chamber and the poppet chamber.